This is a theme change, not something you can toggle in MATE Tweak. Also, only message dialog button are affected. Linked buttons do make sense in other places and they continue to function normally there

For the visually impaired amongst your users, this is a huge improvement. Being able to see where one button starts and another ends makes a difference! Thank you for listening. It's something UM does really well.

Speaking of that, Discord made the regrettable decision to conjoin the video and voice chat buttons for the video mode beta, with only the pointer illuminating where the cursor is. I can see an argument about how somebody would select one or the other by accident due to this.

I like easily understandable generic buttons. It's what I think a golden category old style thing. Inspirational because old and simple puts focus on life and work. I don't mean the Apple fashion fancy simple of course. My opinion on style changes is why not make the old version available as well every time? Choice always adds extra value to products.

I've just checked this in UM 17.10 and the buttons are still showing the older design for themes such as OSX Arc, I'm assuming this is only specific to certain themes that have this change implemented?